Britain's gold-plated triple AAA status came under pressure today after an influential credit ratings agency put it on negative watch.

Standard & Poor's (S&P) said there was a one in three chance it could lower Britain's rating within the next two years, if economic conditions weaken in the UK.

It said it expected government debt as a percentage of gross domestic product (GDP) to continue to rise in 2015, before declining again, with future employment or growth shocks putting further pressure on government finances.

The agency said: "In our opinion, many of the factors that have restrained growth in recent years will likely continue to do so in the near term."

A Treasury spokesman said the move brought S&P in line with rival agencies Fitch and Moody's, which both revised the UK to a negative outlook earlier this year.

A downgrade by one of the big three credit ratings agencies would drive up the UK's borrowing costs, potentially jeopardising the Government's deficit reduction plans.

The S&P report comes after Chancellor George Osborne said he will not be able to start bringing down national debt as a percentage of gross domestic product (GDP) in 2015/16, in his Autumn Statement.

He said he must extend his fiscal consolidation period by a year to 2017/18 after the independent tax and spending watchdog, the Office for Budget Responsibility (OBR), said it expected GDP to fall this year by 0.1%, compared to previous estimates of 0.8% growth.

Sky's economics editor Ed Conway said: "It brings Standard and Poor's into line with the other ratings agencies, they all now say that although the UK is officially a AAA rated sovereign debt issuer, they've put a negative outlook on it.

"Essentially that means that it's an official warning there's a chance that it could get downgraded within the next 18 months to two years.

"It'll be a concern for the Chancellor given that it has come just after the Autumn Statement.

"Standard and Poor's citing mainly the fact the UK's debt is continuing to rise to a level that it would see as being inconsistent with a AAA rated country.

"It does seem that within the Treasury there's a growing cognisance that it may be difficult to hang onto that AAA rating in the end."